Abstract This paper attempts to clarify the debate between those philosophers who hold that the development of science is governed by objective standards of rationality and those sociologists of science who deny this. In particular it focuses on the debate over the ?symmetry thesis?. Bloor and Barnes argue that a properly scientific approach to science itself demands that an investigator should seek the same general type of explanation for all decisions and actions by past scientists, quite independently of whether or not she or he happens to agree with those decisions or approve those actions as ?correct? or ?rational?. I try to improve on previous treatments of the ?rationalist? position (by Lakatos, Laudan, Newton?Smith and Brown) and clarify the exact asymmetries to which the ?rationalist? is, and is not, committed